High temperature insulation for ceramic matrix composites

ABSTRACT

A ceramic composition is provided to insulate ceramic matrix composites under high temperature, high heat flux environments. The composite comprises a plurality of hollow oxide-based spheres of varios dimentions, a phosphate binder, and at least one oxide filler powder, whereby the phosphate binder partially fills gaps between the spheres and the filler powders. The spheres are situated in the phosphate binder and the filler powders such that each sphere is in contact with at least one other sphere. The spheres may be any combination of Mullite spheres, Alumina spheres, or stabilized Zirconia spheres. The filler powder may be any combination of Alumina, Mullite, Ceria, or Hafnia. Preferably, the phosphate binder is Aluminum Ortho-Phosphate. A method of manufacturing the ceramic insulating composition and its application to CMC substates are also provided.

This invention was conceived under United States Department of EnergyContract DE-FC21-95MC32267. The United States Government has certainrights hereunder.

RELATED APPLICATION

The present invention is related by subject matter to the inventionsdisclosed in commonly assigned application having Ser. No. 09/049,369,filed on Mar. 27, 1998, entitled "Use of High Temperature Insulation ForCeramic Matrix Composites in Gas Turbines," (Attorney Docket Nos.T2-97-26 and T2-97-44), which is incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

The present invention relates generally to high temperature ceramicinsulating materials and applications to ceramic matrix composites.

BACKGROUND OF THE INVENTION

Various insulating materials to be used as coatings have been developedto strengthen the resistance of underlying substrates to increasedtemperatures. Thermal Barrier Coatings (TBCs) are commonly used toprotect a machine's critical components from premature breakdown due toincreased temperatures to which the components are exposed. Generally,TBCs extend the life of critical components by reducing the rate ofmetal waste (through spalling) by oxidation.

A fundamental drawback of TBCs is a limitation in thickness that can beused. This thickness limitation of approximately 0.5 mm is due tomanufacturing-induced residual stresses, prohibitive costs, requiredlife of the TBC material, temperature limit of the TBC, and mismatchesin the coefficients of thermal expansion of the TBC and the substrate.In addition, microstructure of conventional TBCs (those applied by bothair plasma spray and physical vapor deposition) is dictated by processconditions, is limited in versatility, and is prone to dimensional andthermal instability at temperatures greater than 1000° C.

Materials comparable to TBCs are fibrous ceramic insulating materials. Amajor drawback of these materials, however, is that they have lowdensities which lead to very poor erosion resistance. Therefore, fibrousceramic insulating materials are inapplicable to high velocity gas flowapplications.

Monolithic tiles are another material to be used for protecting criticalcomponents in high temperature conditions. These tiles have good erosionresistance and insulating properties, however, they are susceptible tothermal shock damage and catastrophic failure. It is, therefore,desirable to provide insulating materials that can withstand hightemperatures without the use of thermal barrier coatings, fibrousceramic insulating materials, or monolithic ceramic tiles.

Commercially available ceramic matrix composites (CMCs) have manypotential applications in high temperature environments. CMCs, however,are limited in their exposure to temperatures near 1200° C. for longperiods of time. In addition, CMCs cannot be effectively cooled underhigh temperatures (>1400° C.) or high heat flux conditions because oftheir lower conductivity than metals and their limitations in coolingarrangements due to manufacturing constraints. It is, therefore,desirable to provide a material that can be used to insulate moderatetemperature ceramic matrix composites, is also erosion resistant,thermal shock, resistant, and has coefficients of thermal expansionrelatively similar to that of CMCs.

European Patent Office publication No. 007,511,04, entitled "AnAbradable Composition," filed Jan. 2, 1997, discloses a ceramicabradable material that can be used to seal ceramic turbine components.This material, however, purportedly has a high temperature capability ofonly 1300° C.

SUMMARY OF THE INVENTION

A ceramic insulating composition to be used over a higher strength,lower temperature ceramic for application in high temperatureenvironments is provided. The composition comprises a plurality ofhollow oxide-based spheres of various dimensions, a phosphate binder,and at least one oxide filler powder, whereby the phosphate binderpartially fills gaps between the spheres and the filler powders. Thespheres are situated in the phosphate binder and the filler powders suchthat each sphere is in contact with at least one other sphere.

The spheres may be any combination of Mullite spheres, Alumina spheres,or stabilized Zirconia spheres. Each of the Mullite spheres has adiameter in the range of approximately 0.1 to approximately 1.8 mm, andpreferably approximately 0.8 to approximately 1.4 mm. Each of theAlumina spheres has a diameter in the range of approximately 0.1 toapproximately 1.5 mm, and preferably approximately 0.3 to approximately1 mm, and each of the stabilized Zirconia spheres has a diameter in therange of approximately 0.1 to approximately 1.5 mm, and preferablyapproximately 0.8 to approximately 1.2 mm.

When only Mullite spheres are used, the spheres have a weight percentageof 32%±10% of the composition. When only Alumina spheres are used, thespheres have a weight percentage of 63%±15% of the composition. Whenonly stabilized Zirconia spheres are used, the spheres have a weightpercentage of 58%±15% of the composition. In one preferred embodiment ofthe composition, the spheres are 20% Mullite spheres by volume and 80%Alumina spheres by volume

The filler powder may be any combination of Alumina, Mullite, Ceria, orHafnia. In one preferred embodiment of the composition containingmullite spheres and mullite powder, the mullite spheres have a weightpercentage of 32%±10%, the mullite filler powder has a weight percentageof 32%±15%, and the phosphate binder has a weight percentage of 31%±15%of the slurry composition. In this preferred embodiment, when onlyMullite is used as the filler powder, the combination of the phosphatebinder and the Mullite has a viscosity of approximately 9,000centipoise.

A method of manufacturing the ceramic insulating composition of thepresent invention is also provided. The method comprises the followingsteps: (a) mixing raw materials to form a viscous slurry, the rawmaterials comprising a phosphate binder and oxide filler powders; (b)adding a predetermined amount of hollow oxide-based spheres to theslurry to create a slurry mixture; (c) cast the mixture into presoakedmolds; (d) allow the castings, which have a viscosity, to dry; (e) whenthe viscosity of the castings is sufficiently high for "green" bodies tobe extracted from the molds with minimal dimensional distortion, removethe "green" bodies; (f) after the "green" bodies have been removed,recycle the molds by (i) washing out the leached phosphate by running inwater followed by oven drying, and (ii) when fully dry, if the dryweight of the mold is within approximately 1% of the original dryweight, use the mold again to perform another casting; (g) transfer the"green" bodies to a drying oven to remove free water; (h) fire thecasts, evaporating residual free water and thermally transform thephosphate to a refractory bond in the process; and (j) finish machine asrequired.

In a preferred procedure, step (c) of the method of manufacturing theceramic insulating composition of the present invention furthercomprises casting the mixture within approximately 24 hours of beingmade, step (g) further comprises transferring the "green" bodies to adrying oven at approximately 80° C. to remove free water, step (h)further comprises transferring the casts to the firing oven when the"green" bodies become stable, and step (i) further comprises thefollowing steps: (i1) begin firing by slowly heating the firing oven toa temperature of approximately 120° C.; (i2) dwell increasing the firingoven at a temperature of approximately 120° C. until most of the freewater is removed by evaporation; (i3) slowly increase the temperature ofthe firing oven to a temperature of approximately 250° C.; (i4) dwellincreasing the temperature at a temperature of approximately 250° C.until all of the free water is removed by evaporation; and (i5) slowlyincrease the temperature of the firing oven to a temperature ofapproximately 1200° C. to form a refractory phase of the phosphate.

In another preferred procedure, step (e) further comprises a step afterextraction from the molds, of shaping the "green" bodies to conform tothe contour of a mating substrate surface. If performed, this step willachieve near net shaping capability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged perspective view of a preferred embodiment ofceramic insulating coating according to the present invention.

FIG. 2 is a further enlarged perspective view, depicting a crosssection, of a preferred embodiment of ceramic insulating coatingaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a material that uses high temperatureceramic material as an insulator over a higher strength, lowertemperature ceramic for application to high temperature environments.Referring to the drawings, there is shown in FIG. 1 an enlargedperspective view of a preferred embodiment of ceramic insulatingcomposition 10 (or coating 10) according to the present invention. Thisview also shows a cross section of ceramic insulating coating 10 placedon a substrate 8 of ceramic matrix composite and kept in place with alayer of adhesive 9.

FIG. 2 shows a further enlarged perspective view, depicting the crosssection, of a preferred embodiment of ceramic insulating coating 10according to the present invention. The coating 10 comprises hollowoxide-based spheres 20 of various dimensions in a combination 12 of aphosphate binder and various oxide filler powders. The phosphate binder"bridges" the gaps between the spheres 20 and the oxide filler powders.The spheres 20 are manufactured at high enough temperatures to make themstable at 1600° C., depending on the particular composition of thecoating 10. Firing temperatures up to 1600° C. are used to create thecoating, which is dimensionally stable, chemically stable and erosionresistant.

Dimensional stability is primarily controlled by the spheres 20. Thecoating 10 achieves improved erosion resistance by introducing closedporosity on a macroscopic scale with a relatively dense arrangement ofspheres 20. Preferably, the spheres 20 are arranged so that each sphereis in contact with at least one other sphere 20. More preferably, thespheres 20 are arranged so that each sphere is in contact with severalother spheres 20, i.e., at least 3 or 4 spheres 20. This provides theimproved dimensional stability, especially at elevated temperatures near1600° C. Sphere contact such as that present in the coating 10 and theresulting dimensional stability is not achieved by prior art coatings. Acomposition 10 according of the present invention is stable preferablyat temperatures greater than approximately 1300° C., and more preferablyat temperatures up to approximately 1600° C.

Oxide filler powders in combination with the phosphate binder can bevaried to control properties of the coating 10. Specific coating systemsmay be formulated to cover a range of coefficients of thermal expansion(CTE). As understood by those of skill in the art, the CTE of thecoating 10 must be as close as practically possible to the CTE of thesubstrate 8 for the coating 10 to remain in place on the substrate 8.Various properties of exemplary coatings 10, A and B, are shown in Table1.

                  TABLE 1                                                         ______________________________________                                        Material            A       B                                                 ______________________________________                                        Use Temp (° C.)                                                                            1200    1600                                              CTE (×10.sup.-6 mm/mm ° C.)                                                          5.85    5.85                                              Thermal Conductivity                                                                              1.27    2.21                                              (W/mK) at 1400° C.                                                     Erosion Resistance* 7.5     4.5                                               (g/kg) at 1100° C.                                                     ______________________________________                                         *Tested at 15° impingement angle, 900 ft/s erodent speed.         

Material properties such as thermal conductivity and erosion resistancecan be tailored by specific selection of filler materials or spherecompositions. The hollow oxide-based spheres 20 of the coating materialof the present invention can be made of either Mullite, Alumina,stabilized Zirconia (usually Yttria stabilized Zirconia) or anycombination thereof. The preferred range of diameters of the Mullitespheres is approximately 0.4 to approximately 1.8 mm, and morepreferably approximately 0.8 to approximately 1.4 mm. The preferredrange of diameters of the Alumina spheres is approximately 0.3 toapproximately 1 mm. The preferred range of diameters of the stabilizedZirconia spheres is approximately 0.6 to approximately 1.2 mm, and morepreferably approximately 0.8 to approximately 1 mm.

When only Mullite spheres are used, i.e., KCM Holospheres® manufacturedby Keith Ceramics, Inc. of Great Britain, the preferable weightpercentage of spheres 20 in the coating 10 is 32%±10%, more preferably32%±5%, and even more preferably approximately 32%. When only Aluminaspheres are used, i.e., manufactured by Ceramic Fillers, Inc. ofAtlanta, Ga., the preferable weight percentage of spheres 20 in thecoating 10 is 63%±15%, more preferably 63%±10%, even more preferably63%±5%, and most preferably approximately 63%. When only stabilizedZirconia spheres are used, i.e., manufactured by Keith Ceramics, Inc.,the preferable weight percentage of spheres 20 in the coating 10 is58%±15%, more preferably 58%±10%, even more preferably 58%±5%, and mostpreferably approximately 58%.

To tailor a particular coating to obtain a particular CTE to "match" theCTE of the intended substrate 8, one merely varies the combination ofspheres 20. For example, monolithic stabilized Zirconia spheres have thehighest CTE (approximately 10×10⁻⁶ mm/mm° C.), monolithic Mullitespheres have the lowest (approximately 5.7×10⁻⁶ mm/mm° C.), andmonolithic Alumina spheres have an intermediate value (approximately8.0×10⁻⁶ mm/mm° C.).

A preferred combination of spheres 20 is 20% Mullite and 80% Alumina byvolume. As displayed in Table 2, this sphere composition yields a %linear change of 0.5972, which "matches" a value of 0.5934 for CompositeA (an oxide-based CMC material) and a value of 0.6031 for composite B.For Composite C (a high silica containing oxide-based compositematerial), an all mullite sphere composition is preferred.

                  TABLE 2                                                         ______________________________________                                                                         Oxide/Oxide                                                                   Substrate                                                                     Composition                                  Sphere   Volumetric % Linear Change                                                                            (% Linear Change                             Composition                                                                            Ratio      at 1000° C.                                                                         at 1000° C.)                          ______________________________________                                        Mullite  100        0.5657       0.5631 (C)                                   Mullite and                                                                            50/50      0.5660                                                    Stabilized                                                                    Zirconia                                                                      Mullite and                                                                            50/50      0.5763                                                    Alumina                                                                       Mullite and                                                                            20/80      0.5972       0.5934 (A) and                               Alumina                          0.6031 (B)                                   Mullite and                                                                            10/90      0.6210                                                    Alumina                                                                       Mullite and                                                                             5/95      0.6337                                                    Alumina                                                                       Alumina  100        0.6380                                                    Stabilized                                                                             100        0.7325                                                    Zirconia                                                                      ______________________________________                                    

The oxide filler powders can be Alumina, Mullite, Ceria, Hafnia or anycombination thereof. Preferably, Alumina or Mullite is used as thefiller powder, and most preferably, Mullite is used because of itssuperior high temperature properties. Preferably, when Mullite is used,the weight percentage of the oxide filler powder in the coating 10 is32%×10%×15%, more preferably 32%×10%, even more preferably 32%×5%, andmost preferably approximately 32%. The preferred weight percentages ofthe oxide filler powders vary because of the different atomic mass andparticle size of each.

Preferably, the phosphate binder is Aluminum Ortho-Phosphate in a weightpercentage of 31%±15%, more preferably 31%±10%, even more preferably31%±5%, and most preferably approximately 31%. Preferably, a combinationof Aluminum Ortho-Phosphate binder and Mullite filler powder has aviscosity of approximately 9,000 centipoise, measured with a Brookfield®RV viscometer having a spindle No. of 7 and a rpm of 20.

The manufacturing process for the coating 10 of the present inventioncomprises the following steps: (1) mixing a slurry, (2) casting theslurry, (3) controlled drying, (4) removal of the "green" body, (5)firing, and (6) machining. The mixture is formulated such that the endproduct possesses a CTE practically identical to that of the CMCsubstrate 8.

The process starts with the mixing of raw materials to form a viscousslurry and is accomplished in two stages. First, AluminumOrtho-Phosphate and the filler powder is mixed to an exact formulationof 50% aqueous solution of Aluminum Ortho-Phosphate and is storedair-tight (with a shelf life of up to 2 months). Alter-natively, one canstart with a 50% aqueous solution of Aluminum Ortho-Phosphate.

When a casting is performed, exact amounts of hollow spheres 20 areadded to the slurry and the slurry mixture is then cast withinapproximately 24 hours of being made. The slurry containing the hollowspheres 20 is cast into presoaked molds. The molds are presoaked withdeionized water prior to casting to allow the capillary drying of thecasting to be effective. If the slurry was cast into a dry mold, waterfrom the cast would be extracted too quickly into the mold creating adry surface on the casting preventing further controlled drying fromoccurring. This would result in an non-homogenous end product. At acritical stage in the drying of the castings, the viscosity issufficiently high for the "green" bodies to be extracted from the moldswith minimal dimensional distortion ("green" body is the term used forthe composition prior to firing).

After removal from the mold, the "green" bodies are carefullytransferred to a drying oven (at approximately 80° C.). In a preferredprocedure, before drying, the "green" bodies are shaped to conform tothe contour of a mating substrate surface. This step will achieve nearnet shaping capability. After drying, the "green" bodies are thentransferred to the firing oven. During firing, a slow heating rate isused with a dwell at approximately 250° C. which ensures that all of thefree water is removed by this stage.

Between approximately 250° C. and approximately 565° C., steadydehydration of the phosphate commences and this is controlled by a slowheating rate through this temperature range. The rest of the firingcycle is dedicated to chemical changes in the phosphate structure.Incorrect procedure for removing water from this material system willresult in a defective and weak microstructure.

The molds are recycled after the "green" bodies have been removed. Thisis achieved by washing out the leached phosphate with running waterfollowed by oven drying. When fully dry, the dry weight of the mold mustbe within approximately 1% of the original dry weight in order for themold to be used again. It can be expected to reuse a mold up to 12times.

In preparation for firing, the "green" bodies can be stacked whichminimizes furnace space. The resulting simplified firing cycle is shownin Table 3.

                  TABLE 3                                                         ______________________________________                                        Step   Start Temp                                                                              Ramp Rate  Hold Temp                                                                             Dwell Time                                Number (° C.)                                                                           (° C./min)                                                                        (° C.)                                                                         (mins)                                    ______________________________________                                        1      80        1           250     60                                       2      250       3          1600    240                                       3      1600      10         ambient END                                       ______________________________________                                    

The final phase of the manufacturing process is to machine theinsulating coating 10.

This manufacturing process allows molds to be reused by virtue of theremoval of green bodies at a low temperature stage. This features yieldsthe following benefits: the reduction of both raw materials andmachining, reduced waste, more flexibility in the manufacturing process,"green" preforms can be bonded together to make more complex shapesand/or the attachment of different compositions, the reduction ofcapital equipment, a reduction in furnace space for firing by priorremoval of mold material results in smaller furnaces with less costlyinitial outlays and running costs, reduction of manufacturing cycletime, removal of "green" bodies from the molds allows recasting to beconcurrent with firing and removes the need for labor intensive breakoutof fired casts from spent molds, and simplified firing cycle makesprocessing easier to duplicate.

At temperatures up to approximately 750° C. the phosphate binder mayexist in a glassy form, which is compliant during the firing process.This may provide the potential for shape forming during the firstfiring. By firing the material up to temperatures of approximately 1200°C., a phosphate "bridge" is produced that gives a compliant matrix thatcan be used as a displacement type abradable seal.

By heat treating further to approximately 1600° C., the phosphate"bridge" network that connects the constituents of the material system(the particles and spheres) is significantly modified to form morelocalized and densified phosphate agglomerations within themicrostructure. A material system with new properties results from thischange that retains up to 80% of its room temperature strength at 1400°C., has similar thermal conductivity and excellent erosion resistance(approximately a factor of 2 times better than currently available TBCsystems used on metallic substrates).

The material is fired stand alone and then ground to shape prior tobonding to the substrate 8. The adhesive 9 will vary according to thesubstrate 8. Direct coating onto the substrate 8, however, is alsopossible utilizing the substrate 8 and/or in-situ curing in theapplication environment.

Potential applications for the ceramic insulating coating 10 of thepresent invention are vast. Such applications would include, but notlimited to, high heat flux environments such as those occurring in gasturbine hot section components or re-entry vehicle surfaces. Thesecoatings can be applied to a wide variety of substrate materialsincluding, but not limited to, oxide matrix composites (e.g., Mullite,Aluminosilicate and Alumina), Silicon Carbide matrix composites (made bytechniques such as Chemical Vapor Infiltration or melt-infiltration),Silicon Nitride matrix composites (made by means such as reactionbonding, nitriding, hot pressing or pressureless sintering).

Application of the coating 10 may be performed by forming the coating 10separately and subsequently bonding the coating 10 to the substrate 8using Aluminum Phosphate-based adhesives (or other ceramic-basedadhesive systems) cured at intermediate temperatures, i.e., around 800°C.-1000° C. Coatings of mullite or alumina may be applied to thesubstrate 8 prior to bonding to prevent fiber damage during curingand/or to facilitate bonding. These coatings are especially desirablewhen bonding to non-oxide substrates 8.

It is to be understood that even though numerous characteristics andadvantages of the present invention have been set forth in the foregoingdescription, together with details of the structure and function of theinvention, the disclosure is illustrative only. Accordingly, changes maybe made in detail, especially in matters of shape, size and arrangementof parts within the principles of the invention to the full extentindicated by the broad general meaning of the terms in which theappended claims are expressed.

We claim:
 1. A ceramic insulating composition comprising:a plurality ofhollow oxide-based spheres of various dimensions; a phosphate binder;and at least one oxide filler powder, whereby said phosphate binderpartially fills gaps between said spheres and said filler powders;whereby said spheres are situated in said phosphate binder and saidfiller powders such that each sphere is in contact with at least oneother sphere.
 2. The composition of claim 1 stable at temperatures up toapproximately 1600° C.
 3. The composition of claim 1 having acoefficient of thermal expansion in the range of approximately 5.7×10⁻⁶mm/mm° C. to approximately 10×10⁻⁶ mm/mm° C.
 4. The composition of claim1 having thermal conductivity less than approximately 2.5 W/km.
 5. Thecomposition of claim 1 having an erosion 20 resistance in the range ofapproximately 4.5 g/kg to approximately 7.5 g/kg, when measured at 1100°C., a 15°impingement angle, and an erodent speed of 900 ft/s.
 6. Thecomposition of claim 1, wherein spheres are selected by one or more ofthe following:Mullite spheres; Alumina spheres; and stabilized Zirconiaspheres.
 7. The composition of claim 6, wherein:each of said Mullitespheres has a diameter in the range of approximately 0.4 toapproximately 1.8 mm; each of said Alumina spheres has a diameter in therange of approximately 0.3 to approximately 1 mm; and each of saidstabilized Zirconia spheres has a diameter in the range of approximately0.6 to approximately 1.2 mm.
 8. The composition of claim 1, wherein saidspheres are one of the following:Mullite spheres, wherein said sphereshave a weight percentage of 32%±10% of the composition; Alumina spheres,wherein said spheres have a weight percentage of 63%±15% of thecomposition; and Stabilized Zirconia spheres, wherein said spheres havea weight percentage of 58%±15% of the composition.
 9. The composition ofclaim 1, wherein said spheres are 20% Mullite of said spheres by volumeand 80% Alumina of said spheres by volume.
 10. The composition of claim1, wherein said filler powder is one or more of the following:Alumina;Mullite; Ceria; and Hafnia.
 11. The composition of claim 1, wherein saidfiller powder is Mullite filler powder.
 12. The composition of claim 11,wherein said oxide filler powder has a weight percentage of 32%±15% ofthe composition.
 13. The composition of claim 1, wherein said phosphatebinder is Aluminum Ortho-Phosphate.
 14. The composition of claim 13,wherein said Aluminum Ortho-Phosphate has a weight percentage of 31%±15%of the composition.
 15. The composition of claim 13, wherein said fillerpowder is Mullite and said combination of said phosphate binder and saidMullite has a viscosity of approximately 9,000 centipoise.